CT systems and methods are widely used, particularly for medical imaging and diagnosis. CT systems generally create images of one or more sectional slices through a subject's body. A radiation source, such as an X-ray tube, irradiates the body from one side thereof. A collimator, generally adjacent to the X-ray source, limits the angular extent of the X-ray beam, so that radiation impinging on the body is substantially confined to a planar region defining a cross-sectional slice of the body. At least one detector (and generally many more than one detector) on the opposite side of the body receives radiation transmitted through the body substantially in the plane of the slice. The attenuation of the radiation that has passed through the body is measured by processing electrical signals received from the detector.
Typically, in commonly-used third- and fourth-generation CT scanners, the X-ray source (or multiple sources) is mounted on a gantry, which revolves about a long axis of the body. In third-generation scanners, the detectors are likewise mounted on the gantry, opposite the X-ray source, while in fourth-generation scanners, the detectors are arranged in a fixed ring around the body. Either the gantry translates in a direction parallel to the long axis, or the body is translated relative to the gantry. By appropriately rotating the gantry and translating the gantry or the subject, a plurality of views may be acquired, each such view comprising attenuation measurements made at a different angular and/or axial position of the source. Commonly, the combination of translation and rotation of the gantry relative to the body is such that the X-ray source traverses a spiral or helical trajectory with respect to the body. The multiple views are then used to reconstruct a CT image showing the internal structure of the slice or of multiple such slices, using conventional methods. The lateral resolution of the CT image, or specifically, the thickness of the slices making up the image, is generally determined by the angular extent of the radiation beam or of the individual detectors, whichever is smaller.
Traditionally, adaptive noise reduction techniques are used to remove the noise. However, those techniques generate a non-linear relationship between the image noise and the X-ray tube current.